048 



CILIA. 



Fig. 299. 



A. Cross section of the Aphrodita aculeata. 



B. Alimentary canal and cceca, seen from above. 



Jhe cells and scales, inclosing them in a space 

 (e) to which the water has free access. Re- 

 turning to the abdomen, we find the nearly 

 straight alimentary canal, its anterior third 

 (f, Jig. B) forming the stomach, the remaining 

 part or intestine fg t jig. A and B) being fur- 

 nished on each side with a number of long 

 caca fh), whose branched extremities (i, i) 

 are in part lodged in the before-mentioned 

 cells. The abdomen is lined with a de- 

 licate peritoneal membrane, which also lines 

 the cells, and is reflected over the viscera. 



In the living Aphrodita the water freely 

 enters and issues from the space (e} beneath 

 the felty membrane, passing over the external 

 surface of the cells and their appendages. The 

 flow of the water in this passage is produced, 

 as I have repeatedly observed, by the elevation 

 and depression of the scales, and on no part 

 of the surface over which the fluid passes is 

 the ciliary motion to be observed. But the 

 water also enters the cavity of the abdomen, 

 though it is doubtful by what orifices this takes 

 place, for my endeavours to find those de- 



scribed by Treviranus* in the alternate in- 

 tervals of the feet have never been successful. 

 In whatever way it may happen, however, 

 there can be no doubt of the fact that the 

 water enters the abdomen, and consequently 

 fills the dorsal cells and surrounds the intestine 

 and its cceca, which last organs, according to 

 Sir Everard Home and Treviranus, exercise a 

 respiratory function, an opinion which derives 

 additional probability in considering the phe- 

 nomena of the ciliary motion to be here de- 

 scribed. The ciliary motion exists in two 

 situations, 1st, on the external surface of the 

 intestine and cceca and the internal surface of 

 the cells, which surfaces are in contact with 

 the contained water ; 2dly, within the intes- 

 tine and cceca, or on their internal surface. The 

 motion as usual persists for some time in de- 

 tached parts, and the direction of the currents 

 is constant. On the intestine the currents pass 

 from the inferior surface round the sides to the 

 upper part (as marked by the arrows). On 

 the cceca the direction is outwards or towards 

 the cells, and the motion is very distinct at 

 their extremities. The direction on the inner 

 surface of the cells was not completely made 

 out, but it seemed to be chiefly downwards. 

 Nor was the direction of the impulsion satis- 

 factorily ascertained on the internal surface 

 of the intestine and cceca, though of the ex- 

 istence of the phenomenon in that situation 

 there could be no doubt. 



From what has been stated, it appears then, 

 first, that in the Aphrodita the water finds 

 access to the outside of the cells, over which 

 it is conveyed by the elevation and depression 

 of the dorsal scales, and to the inside of the 

 cells, over which, as well as over the external 

 surface of the intestine and its ccecal appen- 

 dages, it is moved by the action of cilia. In 

 both situations the motion of the fluid is pro- 

 bably subservient to the respiratory function, 

 and if it really be so, we must reckon the 

 scnles, the cells, the alimentary canal, and its 

 appendages, as constituting the respiratory ap- 

 paratus. Secondly, that the ciliary motion 

 exists also on the internal surface of the in- 

 testine and cceca, where it is likely connected 

 both with respiration and digestion. In all 

 this we cannot overlook the analogy which 

 subsists between the Aphrodita and Asterias. 

 In both the water is conveyed, though by a 

 different mechanism, over the external surface 

 of the body ; in both it enters the cavity con- 

 taining the viscera ; in both it is moved along 

 the parietes of the cavity and surface of the 

 viscera in a determinate direction by the 

 agency of cilia ; and, lastly, in both the ciliary 

 motion occurs on the internal surface of the 

 digestive organs. 



I first observed the ciliary motions in the 

 Aphrodita aculeata in 1830, at the same time 

 with the late Mr. Cheek, who gave notice 

 of the fact in the journal of which he was 

 conductor ;f but most of the observations on 



* Zeitschrift fur Physiologic, Band iii. p. 158. 

 t Edin. Jour, of Nat. and Geog. Science, April, 

 1831, p. 246. 



